An automatic television monitoring system of the foregoing type is described in the German printed application DT-OS No. 19 13 768, wherein a television camera is focused on an object or scene to be protected to produce an "actual" video signal. A comparison image store or image signal representing the monitored object or scene in its original or desired state is stored at an earlier time in a storage unit to produce a "desired" or original video signal. The video signals from the television camera and the comparison image store are compared in a difference signal detector. The difference signal which is generated will be equal to zero when no change in the scene scanned by the television camera has taken place. However, if a change has taken place the "actual" video signal will differ from the "desired" video signal, making the difference signal greater than zero. As soon as the signal difference at the output of the signal difference detector exceeds a pre-selected threshold value for a pre-selected period of time, an alarm signal is given.
When the object or scene protection is extended to larger objects, scenes or several objects distant from each other, the foregoing type of monitoring scene requires several television cameras, a corresponding number of comparison image stores and a corresponding number of difference signal detectors. The system thus becomes more expensive and increases in cost in proportion to the number of television cameras used. Because of the limited monitoring area of an individual television camera, there is often a need for using several television cameras. Substantial systems costs arise in this case, especially as the result of the large number of comparison image stores. To overcome this disadvantage, it is proposed in the German printed application DT-AS No. 24 11 634, on the one hand, to transmit the video signals of the individual television cameras by way of a time-controlled time-division multiplex (TDM) to the input of a single information store, and, on the other hand, also to transmit the video signals in time-division multiplexing fashion through a second TDM electronic switch to a first input of a comparison device. In this case, all television cameras are synchronized by a synchronizing scanner, which is contained in the information store, in such a manner that a frequency and phase synchronization of the television cameras is constantly assured, and, in addition, the second TDM electronic switches are so controlled through special characteristic signals, which are also stored in the information store, that, at the two inputs of the comparison device, an actual video signal of a certain television camera on the one hand, and a video signal of this television camera, which is temporarily stored in the information store and which is therefore time-delayed, on the other hand, are always simultaneously present. However, the synchronization of several television cameras from one central control is relatively expensive. In addition, unintended false alarms can be triggered by synchronization errors in this monitoring system.